The present invention relates to current detectors and, more particularly, to a detector circuit for sensing current flow in an utilization circuit that provides load current therefrom and which is coupled to the detector circuit wherein the detector circuit provides a current output that is proportional to the load current.
Myriad of applications exists for detector circuits. One application of a current detector may be to control the operating state of high output current operational amplifiers. In present day applications it is a desired goal to produce amplifiers with low quiescent power consumption but such prior art amplifiers have been deficient in output drive current and alternating current (AC) performance. In fact, it is still desirous to provide an operational amplifier that has high drive current capability in order to drive low impedance loads while also having minimum bias drain current to reduce power consumption. Indeed, in portable battery powered applications such as mobile telephones, consumer entertainment systems e.g., radios and video games etc., it is especially important to limit the quiescent power consumption of amplifiers used in such applications.
Most, if not all, present day high output current operational amplifiers are comprised of an input stage that is coupled to an output stage. In response to an alternating input signal being applied to the input stage, the operational amplifier both sources and sinks drive current to and from a load coupled to the output stage. Typically, the input and output stages are biased to a quiescent drain current to permit quality audio and data processing applications. For example, low power, high output current amplifiers, such as the MC 33178 manufactured by Motorola, Inc., draw approximately 420 microamperes of drain current per amplifier in the quiescent operating mode with no input signal applied thereto. For micropower and battery powered applications there is a need to reduce drain current requirements over the foregoing while providing amplifiers that can supply high load currents. One manner in which the foregoing may be accomplished is to utilized circuitry for sensing when the input signal is below a certain threshold value to minimize the current drain of the amplifier and once the input signal exceeds this threshold to increase the bias to the stages so that the amplifier operates in its intended range. One method to do this is to sense the absence or presence of load current in the output stage of the amplifier. The absence of load current can then be detected to place the amplifier in a low quiescent bias current drain state. In response to detection of load current, due to an applied input signal, the bias current is then increased as described above.
Hence, there is a need for an improved method and circuit that can provide an output signal which may be utilized to control a load by providing a load control signal when a detected current exceeds a predetermined threshold value.